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I ran across this really interesting blog (via Tyler Cowen's blog) post on nitrogen use in agriculture by Adam Merberg, who says he is a "reformed food reformer."

Merberg's main point is that, aside from a small handful of crops, nitrogen is a key limiting ingredient in growing more food. Much of the nitrogen used in modern commercial agriculture comes from the air! Well, it's taken out of the air by a "synthetic" process that prohibits its use in organic agriculture.

Organic agriculture, like all agriculture, requires nitrogen. Soil (regardless of how you farm) looses fertility after a while and requires replenishment to continue productivity. Organic seeks nitrogen in cover crops (like clover and soy) but mainly through use animal manure. Merberg asks a reasonable question: where does the manure get it's nitrogen. The answer is that it largely comes through conventional agriculture and the "synthetic" process invented by Haber. The manure spread on many organic farms comes from cows/hogs/chickens that ate grains grown using "synthetic" fertilizer.

When we read that organic can "feed the world" we need to ask where all the nitrogen will come from to make it happen.

Here are a few snippets from Merber's article (the back-and-forth in the comments was interesting too):

By identifying manure as a source of nitrogen, Vasilikiotis dodges the issue of nitrogen fixation entirely. However much nitrogen exists in manure today, much of it has been fixed industrially before being taken up by corn plants and laundered through the guts of conventionally-farmed animals. Vasilikiotis does not explain how that manure might come to be in an organic world. To do so would require demonstrating the potential for sufficient biological nitrogen fixation

and

Natural processes, like atmospheric nitrogen deposition, can help replenish some nutrients, but the fact remains that the nutrient cycle remains open. Maintaining modern yields generally requires inputs of some kind to replace nutrients removed in crops. For instance, Joel Salatin’s Polyface Farm–which Pollan’s The Omnivore’s Dilemma identifies as a model for sustainable agriculture and describes as “completely self-sufficient in nitrogen”–actually brings in nitrogen in conventionally-grown grain, which is fed to chickens whose manure fertilizes the pasture.

and, interestingly . . .

In recent years, the US government has begun allowing the recycling of human waste by authorizing the use of treated sewage sludge, called biosolids, as fertilizers. However, in 1998 organic advocates successfully protested proposed guidelines which would have allowed application of biosolids in organic agriculture. Whatever the merits of their objections, it is ironic that the movement for a more “natural” agriculture now opposes ending the waste of nutrients that Liebig once decried as “a sinful violation of the divine laws of Nature.”

For the record, I'm not against organics. But, I am against the mis-truths that are often spread in their defense.